In commonly assigned U.S. Pat. Nos. 5,920,287; 5,995,046; 6,121,926; and 6,127,976, and published patent application Nos. 2001/0030625, 2002/0118655, and 2002/0181565, the disclosures which are hereby incorporated by reference in their entirety, a real time location system is disclosed using a plurality of tag emission (signal) readers that are disbursed throughout a monitored environment, such as a manufacturing environment. The tag signal readers are operative with a location processor to determine which tag signals transmitted from tag transmitters are first-to-arrive signals. The location processor conducts differentiation of the first-to-arrive signals to locate a tag within the monitored environment.
These location and tracking systems provide asset management that not only addresses the need to locate and track different components in the course of their travel through a manufacturing and assembly sequence, but also addresses the more general problem concerning component and equipment inventory control and allow control concerning the whereabouts of any and all assets of a business, factory, educational, military, or recreational facility. In some systems, status information can be provided to a tag transmitter by means of sensors associated with the tag. Typically, the tag signal readers are placed at known locations throughout the environment having the objects to be located or tracked. The system uses time-of-arrival differentiation of repetitive spread spectrum, short duration pulse “blinks” from object-attached tags. The system provides the practical, continuous identification of the location of each and every object within the environment of interest, irrespective of whether the object is stationary or moving.
Much of the disclosure in these incorporated by reference patents and published patent applications rely on differential time-of-arrival (TOA) applications that receive a transmitted signal at multiple physical locations corresponding to the receiver to spatially locate the tag transmitter. Normally, the clocks at each receiver are synchronized so that differences in the indicated time-of-arrival closely approximate the actual time-of-arrival difference. In order to establish this system, each receiver is operative as a network node and must understand and adjust to a synchronization signal. Those disclosures, however, do not adequately describe techniques to determine differences in signal arrival times, for example, if the receivers are unsynchronized.